The sexually dimorphic vasopressin system of the rat may serve as an informative in vivo model for elucidating the cellular and molecular mechanisms controlling neuronal development. Male rats possess two to three times the number of vasopressin neurons than do females in the bed nucleus of the stria terminalis and the medial amygdala (BST/MA). Gonadal steroid hormone levels during a perinatal period determine the number of vasopressineric neurons in adulthood in these areas. This proposal first aims to clarify whether or not galanin-only production is a "default" phenotype to vasopressin and galanin coexpression. I will do this by developing a cell birth profile for galaninergic neurons and comparing this profile with that generated for vasopressinergic neurons (Specific Aim 1). Secondly, using in situ hybridization for vasopressin and galanin mRNA, I will determine which steroid hormone is critical in the sexual differentiation of this system (Specific Aims 2 and 3). Finally, I will use differential display polymerase chain reaction (ddPCR) to isolate genes in the BST/MA that may be part of a steroid-regulated pathway that initiates vasopressin production (Specific Aim 4). These studies will not only contribute to the understanding of how steroid hormones affect neural and behavioral development, they may succeed in identifying molecular components involved in steroid-dependent neuropeptide expression.